In the semiconductor production industry, various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include the deposition of layers of different materials including metallization layers, passivation layers and insulation layers on the wafer substrate, as well as photoresist stripping and sidewall passivation polymer layer removal. In modern memory devices, for example, multiple layers of metal conductors are required for providing a multi-layer metal interconnection structure in defining a circuit on the wafer. Chemical vapor deposition (CVD) processes are widely used to form layers of materials on a semiconductor wafer.
Silicon nitride has been an important material in various semiconductor applications. For instance, silicon nitride has been used as a mask against oxygen diffusion during a local oxidation (LOCOS) process; as a passivation layer for its superior barrier property to contaminants; as a gate dielectric layer in memory devices; and as an interlevel dielectric layer in an oxide-nitride-oxide (ONO) stacked-gate structure. Silicon nitride also has superior barrier properties against metal ions and moisture.
Silicon nitride has been widely used as a passivation layer for protecting a semiconductor component. Silicon nitride can be formed by either a LPCVD or PECVD technique. The LPCVD technique, where dichiorosilane is used as the reactant gas, can be carried out in a hot-wall LPCVD system, such as in a vertical furnace. The chemical reaction can be described as follows:3SiH2Cl+10NH3→Si3N4+6NH4Cl+6H2
The hot-wall LPCVD system is normally carried out at a temperature between about 750°˜800° C., and the chamber pressure is kept at several hundred m Torr. A layer of stoichiometric silicon nitride can thus be deposited on a wafer surface. A typical deposition equipment utilizing a vertical furnace is shown in FIG. 1.
During a vertical furnace silicon nitride process, as described by the above mechanism for the chemical reaction, a reaction by-product such as ammonium chloride (NH4Cl) in the form of a fine powder can easily deposit on any cold surface in the furnace or in the ducting system for the furnace. The ammonium chloride powder must be captured by a cold trap such that it does not form on the inner walls of the ducting system or in the furnace and present a serious contamination source. For instance, fine powder in the ducts may be syphoned back into the furnace during a deposition process if the pressure in the furnace is not carefully controlled. The capture efficiency of the cold trap for the ammonium chloride fine powder is therefore an important factor in the successful deposition of silicon nitride films in a furnace technique.
As shown in FIG. 1, a vertical furnace unit 12 is the heart of a silicon nitride deposition system 10. During the deposition of a silicon nitride film on a plurality of wafers 16 positioned in the vertical furnace, the furnace exhaust gas 14, which contains unreacted reactant gases such as dichlorosilane, ammonium and reaction byproduct ammonium chloride powder, is drawn through a cold trap 22, via an exhaust conduit 20, by a vacuum pump 18 before the furnace exhaust gas enters into a gas treatment unit (not shown) and is released into a factory exhaust system (not shown). The capture of substantially all of the ammonium chloride fine powder in the cold trap 22 is therefore an important step in a successful exhaust gas treatment process for depositing silicon nitride.
A schematic view of a typical conventional cold trap 22 is shown in FIG. 2 and includes a bellow or pipe 24 which receives the exhaust gases from the exhaust conduit 20 of the system 10 shown in FIG. 1. A jacket heater 23 on the pipe 24 prevents the temperature of the exhaust gases flowing through the pipe 24 from dropping below a temperature of typically about 150° C. A trap housing 25 is provided in fluid communication with the outlet end of the pipe 24. The exhaust gases flow through a typically constant-diameter flow bore 26 extending through the trap housing 25, and a water cooling coil 27 winds through the flow bore 26. Cooling water is introduced into the water cooling coil 27 through a cooling water inlet 28, and is conducted from the water cooling coil 27 and the trap housing 25 through a cooling water outlet 29. Typically, the cooling water inlet 28 is located at the same vertical position as the cooling water outlet 29, as shown. Accordingly, as the exhaust gases flow through the flow bore 26, the exhaust gases contact the water cooling coil 27, which cools the exhaust gases. Ammonium chloride powder in the exhaust gases become trapped on the water cooling coil 27 and in the flow bore 26, such that the exhaust gases leave the bottom of the trap housing 25 substantially or completely devoid of the ammonium chloride.
One of the problems frequently associated with the conventional cold trap 22 is that the ammonium chloride powder accumulates in the flow bore 26 and on the water cooling coil 27 after a relatively short period of time of operation of the silicon nitride deposition system 10. This tends to restrict or completely block flow of the exhaust gases through the flow bore 26, and consequently increases loading on the vacuum pump 18. Consequently, damage to or failure of the vacuum pump 18 may result. Thus, the cold trap 22 must be subjected to preventative maintenance about every 4 months to remove the ammonium chloride powder from the trap housing 25. This results in unnecessary downtime in operation of the silicon nitride deposition system 10. Accordingly, a new and improved cold trap which is capable of operating for longer periods of time between periodic maintenance is needed.
Accordingly, an object of the present invention is to provide a new and improved cold trap that can be efficiently used in a semiconductor fabrication process for collecting unwanted particles and which does not have the drawbacks or shortcomings associated with conventional cold traps.
Another object of the present invention is to provide a cold trap that can be used effectively in a semiconductor material deposition system such that the cleaning frequency required for the cold trap can be reduced.
A further object of the present invention is to provide a cold trap for use in a semiconductor fabrication process and which does not require frequent cleaning.
Still another object of the present invention to provide a new and improved cold trap which enhances operational efficiency and throughput of a semiconductor fabrication process.
Yet another object of the present invention is to provide a cold trap which may be provided with one or multiple trap plates for trapping particles.
Still another object of the present invention is to provide a cold trap which may be provided with a cone or cup shaped trap core which prevents premature particle blockage of the cold trap.
Yet another object of the present invention is to provide a cold trap which includes a first stage provided with one or multiple trap plates and a second stage which may be provided with a cup- or cone-shaped trap core for preventing premature particle blockage of the cold trap.
A still further object of the present invention is to provide a cold trap for use in a vertical furnace for depositing silicon nitride films wherein the trap has greatly improved efficiency for trapping ammonium chloride fine powder.
Another object of the present invention is to provide a cold trap which is particularly suitable for trapping ammonium chloride or other particles in a semiconductor fabrication process but which may be equally adapted to a variety of other industrial applications.